Valve

ABSTRACT

VALVES FOR INTERNAL COMBUSTION ENGINES ARE CAST OF HIGH CARBON, HIGH CHROMINUM ALLOY CONTAINING FROM ABOUT 1.3% TO ABOUT 3.1% CARBON, FROM ABOUT 15% TO ABOUT 35% CHROMIUM WITH THE REMAINDER IRON, WITH OR WITHOUT UP TO ABOUT 3.25% SILICON, MANGANESE AND OTHER RESIDUALS. AFTER THE METAL IS CAST, IT IS COOLE SO QUICKLY THAT A RELATIVELY SMALL NUMBER OR RELATIVELY LARGE PRIMARY CHROMIUM CARBIDE PARTICLES ARE FORMED AND WIDELY DISPERSED IN A MATRIX OF AUSTENITE CONTAINING A SOLIDE SOLUTION OF CHROMIUM AND CARBON. THEN LARGE NUMBERS OF RELATIVELY SMALL PARTICLES OF CHROMIUM CARBIDES ARE PRECIPITATED FROM THE MATRIX AND DISTRIBUTED THROUGHOUT THE SPACES BETWEEN THE LARGE PRIMARY CARBON PARTICLES LEAVING THE REMAINDER OF THE MATRIX CONTAINING CARBON AND SUSCEPTIBLE TO SUBSEQUENT HARDENING. THEN THE CASTING IS HARDENED BY HEATING AND SUBSEQUENT QUENCHING AT SUCH TEMPERATURE AND FOR SUCH TIME THAT THE MATRIX IS SUBSTANTIALLY CONVERTED TO MARTENSITE WITHOUT SIGNIFICANTLY CHANGING THE CARBIDE PARTICLES.

P 12, 1972 E. A. THOMPSON 3,690,956

v VALVE Original Filed Feb. 24, 1966 2 Sheets-Sheet 1 EARL A THOMPSON J-KZW Attorney S p 12, 1972 E. A. THOMPSON fifiwfififi H VALVE Origifial Filed Feb; .24, 1966 I 2 sheets-Sheet g 1 I EARL A THOMPSON Attorney United States Patent VALVE Earl A. Thompson, Bloomfield Hills, Mich., assignor to F. Jos. Lamb Company, Warren, Mich.

Original application Feb. 24, 1966, Ser. No. 529,830, now Patent No. 3,508,529, dated Apr. 28, 1970. Divided and this application Mar. 18, 1970, Ser. No. 20,515

Int. Cl. C22c 37/06; F011 3/02 US. Cl. 148-2 6 Claims ABSTRACT OF THE DISCLOSURE Valves for internal combustion engines are cast of high carbon, high chromium alloy containing from about 1.3% to about 3.1% carbon, from about to about 35% chromium with the remainder iron, with or without up to about 325% silicon, manganese and other residuals. After the metal is cast, it is cooled so quickly that a relatively small number of relatively large primary chromium carbide particles are formed and widely dispersed in a matrix of austenite containing a solid solution of chromium and carbon. Then large numbers of relatively small particles of chromium carbides are precipitated from the matrix and distributed throughout the spaces between the large primary carbon particles leaving the remainder of the matrix containing carbon and susceptible to subsequent hardening. Then the casting is hardened by heating and subsequent quenching at such temperature and for such time that the matrix is substantially converted to martensite without significantly changing the carbide particles.

This application is a division of my application Ser. No. 529,830 filed Feb. 24, 1966 patented as 3,508,529 Apr. 28, 1970.

This invention relates to internal combustion engine values made of improved high-chromium, high-carbon, iron alloys.

This invention is based in part on my discovery that greatly improved valves can be cast of particular highchromium, high-carbon, iron alloys which can be treated to provide a surprisingly easily machinable casting which can be processed further to give surprising hardness and resistance both to wear and corrosion. The valve is surprisingly stable as to dimension so that it can be machined with customary cutting tools and can be ground to precisely finished size and shape before hardening. Such alloys so treated make surprisingly long lasting, corrosion free, dimensionally stable valves. They are particularly economical and easy to make.

Accordingly one of the objects of my invention is to provide an improved valve which is easily machinable at one stage of its manufacture, and after hardening is highly resistant to wear and corrosion and is dimensionally stable.

Another object is to provide a valve which can be economically made by conventional processes and which has improved dimensional stability.

Another object is to provide an improved process for making precision valves of high resistance to corrosion and wear.

Other objects and advantages of the invention will be understood from the following description and from the annexed drawings in which FIG. 1 is a cross section of a portion of an internal combustion engine showing a valve to which my invention is particularly suitable.

FIG. 2 is an enlarged cross section of a portion of a valve being cast in a mold suitable for casting according to my invention.

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FIG. 3 is a photograph of a polished and etched section of a portion of a casting made of an alloy embodying one form of my invention. This photograph is of metal in the condition as cast, and is magnified about 1495 times. The scale line approximately of an inch long at the bottom of the photograph represents one ten thousandth of an inch (.0001).

FIG. 4 is a photograph corresponding to FIG. 3 of the same alloy after a subsequent heat treatment.

FIG. 5 is a photograph corresponding to FIG. 3 of the same metal after subsequent hardening.

FIG. 6 is a photograph corresponding to FIG. 3 of the same metal after drawing following hardening.

Referring to FIG. 1, 10 designates an engine having a valve 12 which is reciprocated in a bushing or guide 14 pressed in a bore in the engine. The valve stem may have a groove 16 for a spring retainer 18.

My improved valve is cast and one material of which I make some or all of the parts of the valve is a highcarbon, high-chromium, iron alloy containing about 2.20% carbon and about 22.5% chromium. This alloy may also contain about 1.60% silicon and about .90% manganese. The silicon may be added to make the alloy easier to pour. The manganese combines with any sulphur which may be present in the material of which the alloy is made. Also the manganese may improve the hardenability of the matrix of the alloy upon subsequent quenching. Ordinarily such alloys are made from available ingredients including scrap or pig iron of uncertain analysis so that the resulting alloy may contain residual quantities of copper, nickel, molybdenum and other metals. As one example an analysis showed that one batch of my preferred alloy contained 2.20% carbon, 1.60% silicon, .90% manganese, 22.5 chromium, and residuals of .25 copper, 31% nickel and .17% molybdenum.

The silicon, manganese and the residuals amount to about 3.25% and I believe that these do not importantly affect the final metallurgical structure, for the purpose of my invention. Consequently alloys containing them come within my definition of an alloy having stated ranges of carbon and chromium, and having the remainder iron.

I have discovered that alloys of the composition mentioned above, or of the ranges of composition disclosed herein, can be given a new and improved metallurgical structure by cooling quickly after pouring, and that this new metallurgical structure can be treated to provide new, surprising and very desirable properties. As one example a melt having the proportions of ingredients to provide the alloy of the composition set forth above Was poured at about 2750 F. This particular alloy has a liquidus temperature of about 2399 F. and a solidus temperature of about 2270 F., as determined by the Leeds and Northrup carbon determinator.

FIG. 3 is a photograph of a section of a part which has been cast of the above alloy according to my invention. The temperature of the metal has been reduced from the liquidus to the solidus so quickly that two things have happened. One is that the usual formation of primary chromium carbide particles has been arrested, so that the chromium carbide particles formed are fewer in number and smaller than they would be if the metal had cooled slowly. Evidence of this is that the matrix has remained essentially non-magnetic austenite. If the casting had cooled slowly, austenite would not be formed. The other thing that has happened is that the matrix contains large amounts of chromium and carbon in solid solution. Evidence of this is the subsequent formation of very fine chromium carbide particles during subsequent heat treatment. If the casting had cooled slowly the carbon and chromium now remaining in solution would have precipitated out as primary carbides. The primary chromium carbides shown in FIG. 3 are very small, much smaller than if the metal had cooled slowly, and they are more widely dispersed. The largest primary carbide particle visible in FIG. 3, measured in inches is about .00135 long, and in a representative area .001 square there are about 17 primary carbide particles. The large dark particles shown are what is generally called chromium carbides. Among such chromium carbides Cr C and Cr C have been identified. It is also possible for iron to replace some of the chromium to form complex or mixed chromium iron carbides such as (FeCr) C. All of these come within the definition of chromium carbides as that term is generally understood and used herein. The spaces, relatively large with reference to the carbides, are austenite and substantially non-magnetic. The hardness of the alloy as cast is about 44 Rockwell C.

The valve may be cast in any suitable mold such as shown in FIG. 2. A silicon sand shell mold has an upper section 30 and a bottom section 32. The upper section has an enlarged top for receiving molten metal.

The casting, a section of which is shown in FIG. 3, was cooled under the following conditions.

A casting having a wall thickness of about .090 was poured in a silicon sand shell mold having a wall thickness of approximately one and one-half times the thickness of the casting. The mold was at room temperature. The metal was poured at about 2750" F. The cooling rate, under these conditions, dropped the temperature from the liquidus to the solidus so fast that the metallurgical structure shown in FIG. 3 and described above was formed.

I have found that faster cooling forms even smaller and fewer primary chromium carbide particles and slower cooling forms more and larger primary carbides. The thickness of the metal influences the rate of cooling and this influences the metallurgical structure and properties of the cast metal, not only as cast, but in subsequent treatment. For example a thin section cools faster than a thick section. There is an important and discernible difference in the appearance and properties of the metallurgical structures of two thicknesses as cast. Thin sections can also be machined with a high speed steel tool more easily than thick sections after the subsequent heat treating step described below. Also after final hardening, as disclosed below, a thicker section (slowly cooled) is softer than a thinner section (quickly cooled). For example a test casting having a wall .190 thick as cast, cooled as described above, will have an ultimate hardness of about 60 Rockwell C, whereas one of .160 thickness and cooled as described will have a final hardness of about 63 Rockwell C.

The important thing is that the temperature of the metal must be reduced from the liquidus to the solidus so quickly that only relatively small numbers of very small chromium carbides can form, and that they will be formed in an austenite matrix which has large intercarbide spaces in which larger numbers of still smaller chromium carbides can be precipitated upon re-heating, while leaving the matrix containing carbon and in a condition which can be hardened. FIG. 3 shows a typical structure, which has properly cooled according to my invention.

I may affect the cooling in other ways. Since a thick section cools more slowly than a thin section it may be necessary to mold thicker sections in zircon sand, for example, which cools the casting faster than silicon sand. Alternatively chills may be placed in the mold to accelerate the cooling of thick castings, or I may use a permanent mold, water cooled. If the metal cools too slowly the casting will not only be too hard, but it cannot be satisfactorily heat-treated so as to be machinable.

After cooling the casting was heat treated as follows. Its temperature was slowly raised from room temperature to about 1600" F. The time required was three hours. It was held at 1600 F. one hour. It was cooled to about 4 1400 F. during the next 40 minutes. It was cooled to about 1300 F. during the next hour. Total time 5 /3 hours.

FIG. 4 shows a casting after this treatment. It shows that the chromium carbides of FIG. 3 have not changed significantly. The interstices or inter-carbide spaces in the previously austenitic matrix are now substantially filled with a dispersion of very small precipitated chromium carbides, having a representative size of the order of about .000018 (18 millionths of an inch). In a representative area .0001 square there are about 13 of these very small particles, or about 1300 particles in the .001 square containing about 17 primary carbide particles. Thus although the primary chromium carbides in FIG. 3 are very small (a large one being of the order of a thousandth of an inch long) they are of the order of from 50 to times as large as the smaller carbides formed in the re-heating process. The hardness after re-heating was from 27 to 33 Rockwell C.

I do not know the exact nature of the matrix after re-heating, shown in FIG. 4. It is magnetic. It contains carbon, so that it can be hardened by subsequent heat treatment which appears to convert the matrix essentially to martensite having properties typical of tool steel.

In the foregoing heat treatment the time required is a function of temperature, a lower temperature requiring a longer time. Also the time and temperature of this reheating step influences the amount of carbon left in the matrix and so affects the subsequent hardenability of the alloy, when hardened as disclosed below.

This particular combination of carbide particles and the characters of the matrix in the two conditions appear to make possible the machinability at one stage of my invention and the hardness at a subsequent stage, combined with the surprising dimensional stability and other properties I have observed.

After the foregoing re-heating treatment the valve can be machined easily and economically with high speed steel tools and surprisingly can be ground to the exact final shape and desired dimensions. For example the groove 16 can be machined in the stem, the stem can be turned and faced, and if desired ground, to final size and the valve head can be faced.

Thereafter the part may be hardened by holding at a temperature above the critical temperature at which the matrix changes back into austenite and well below the melting point, followed by quenching. The time is a function of temperature, lower temperature requiring longer time. For example the part may be held at about 1750 F. for about twenty minutes, then oil quenched. FIG. 5 shows a casting which has been cooled then re-heated, then hardened as above described. The Rockwell C hardness is about 63 to 65. The two sets of chromium carbide particles have remained unchanged. The matrix has been essentially converted to martensite. I find that this hardening step changes the size of the part so slightly that in the case of articles which are acceptable within tolerances as large as .0001 (one hundred millionths) of an inch, I can grind to final size before hardening. This is of great advantage in manufacturing.

After hardening, the part may be drawn by holding it at a temperature higher than it will ever work in service, for example of about 375 F., for about one hour. The hardness drops about 1 point Rockwell C and the structure is as shown in FIG. 6, with the alloy discussed above.

The advantages of the invention are realized while varying the proportions of the ingredients of the alloy within the limits stated herein. For example I may use carbon up to 2.35% and chromium up to 27.00% without significantly changing, for the purposes of the invention, the characteristics of the alloy from those of the preferred analysis given above.

Increasing the proportion of carbon within certain critical limits tends to increase the final hardness and hence wear resistance of the article. More carbon is re quired in articles having a thick section, because due to slower'cooling, more carbon is combined with chromium, which has a very high affinity-for carbon. If more carbon were not used, the matrix would be so depleted that it could not be hardened satisfactorily. More carbon than about 2.95% appears to render the article impractically diflicult to machine although in some instances I can use up to about 3.10% carbon, particularly with high percentages of chromium. Increasing the proportion of chromium within a wider range of critical limits tends to increase the corrosion resistance and reduction of the chromium content below about 15% appears to reduce the corrosion resistance undesirably. Increasing the proportion of chromium beyond about 30% appears to have no important elfect on either wear or corrosion resistance, except with very high carbon percentages (above 3.10% for example) and increase of chromium beyond about 35% appears to have no advantage and may even be undesirable. There is a desirable relationship between the amounts of carbon and chromium to have the desired effects because one part carbon will combine with about ten parts chromium. Therefore higher proportions of chromium require higher percentages of carbon so as to leave in the matrix, after the re-heating step, enough carbon not combined with chromium, to harden the matrix satisfactorily in the hardening step discussed above.

For example with my preferred alloy first mentioned, the processes described appear to leave about 1.10% of carbon in the matrix after the first re-heating step (in which the smaller carbide particles are formed). Then when the part is hardened as described, the matrix appears to contain no free carbon and is hardened to have properties resembling those of tool steel or 52100 steel. Measurements of properties of the cast and hardened alloy exceed those of steel. For example, a sample of the preferred alloy, cast and treated as above described showed a transverse bending stress of 693,000 pounds per square inch. From this the modulus of elasticity is calculated at 39,000,000. The modulus for steel is about 29,000,000.

Many of the advantages of the invention are present in a range of carbon between 1.70% and 2.85 while using a range of chromium between 15 and 27%.

Valves having different parts requiring different hardness can be made, in part, of an alloy having the general characteristics described above but being even harder and hence even more wear resistant. In such case I may use a carbon content of about 3.10% and may use this with a chromium content varying between about 30% and about 35%. This provides an extremely hard, wear resistant material. It is difiicult to machine by cutting tools, and although it is difiicult to grind I have found that it affords some of the advantages of the invention. By confining this material to the face of the head, for example, I can machine the stem and grind the face of the valve to finished size with minimum grinding. This is partly due to my improved casting process which perm1ts casting of two different metals within very small tolerances, and confines the extremely hard alloy to a small part. It is also due in part to the unusual dimensional stability of the material. This makes it possible to grind to close tolerances and final size before the hardening step of the manufacturing process described above.

An example of such composite valve is shown in FIG. 2 and in my copending application for US. patent Ser. No. 221,115 filed Sept. 4, 1962, and in my British Patent No. 991,513 published May 12, 1965 the disclosures of which are incorporated herein by reference with the same effect as if quoted herein.

In such case the rim 34 of the valve head (the annular part .outside the dotted lines) is cast first, and may be formed of the alloy containing 3.10% carbon and 25.7% chromium mentioned above. The remainder of the head and the entire stem may be formed of any of the other alloys disclosed above. Such as alloy containing 2.20% carbon and 22.5% chromium, or the lower portion 36 of the alloys having the following analyses.

C Si Cu Mn Cr N1 M0 Example:

I claim as my invention:

1. The method of making a valve for an internal combustion engine which includes pouring into a mold a molten iron alloy containing from about 1.30% to about 3.10% carbon and from about 15% to about 35% chromium with the rest iron, rapidly reducing the temperature from the liquidus to the solidus at such a rate that a relatively small number of relatively large primary chromium carbide particles are formed and widely dispersed in a matrix of austenite containing a solid solution of chromium and carbon, then re-heating the cooled valve until large numbers of relatively small particles of chromium carbides are precipitated from the matrix and distributed throughout the spaces between the large primary carbide particles leaving the remainder of the matrix containing iron and susceptible to subsequent hardening, then hardening the valve by heating and subsequent quenching, the last mentioned heating being at such temperature and for such time that the matrix is substantially converted to martensite when quenched without significantly changing the carbide particles.

2. The method of making a valve for an internal combustion engine which includes pouring a molten iron alloy containing from about 1.30% to about 3.10% carbon and from about 15 to about 35% chromium with the rest iron, rapidly reducing the temperature from the liquidus to the solidus at such a rate that a relatively small number of relatively large primary chromium carbide particles are formed and widely dispersed in a matrix of austenite containing a solid solution of chromium and carbon, then re-heating the cooled valve until large numbers of relatively small particles of chromium carbides are precipitated from the matrix and distributed throughout the spaces between the large primary carbide particles leaving the remainder of the matrix containing carbon and susceptible to subsequent hardening, then forming the valve to a predetermined size and shape by removing material from its surface, then hardening the valve by heating and subsequent quenching, the last mentioned heating being at such temperature and for such time that the matrix is substantially converted to martensite without significantly changing the carbide particles.

3. An integrally cast valve for an internal combustion engine at least a part of which is cast of an iron alloy containing from about 1.3% to about 3.1% carbon and from about 15 to about 35 chromium with the remainder iron, said alloy having a minimum hardness of about 61 Rockwell C and having a relatively small number of relatively large primary chromium carbide particles distributed in a matrix of martensite and having a relatively large number of relatively small precipitated chromium carbide particles distributed throughout the matrix between the large primary carbide particles.

4. A valve as defined in claim 3 further characterized by a carbon content of between about 1.7% and about 2.85% and a chromium content of between about 15 and about 27%.

5. A valve as defined in claim 3 further characterized by a carbon content of between about 2.2% and about 2.35% and a chromium content of between about 22% and about 27% 6. A valve as defined in claim 3 further characterized by a carbon content of about 2.2% and a chromium content of about 22.5%

References Cited UNITED STATES PATENTS 8 4/1934 Fink et a1. 123-188 12/1956 Fugua et a1 148-35 X 1/1963 Thompson 148-3 X 4/1940 Berglund 148-35 X OTHER REFERENCES Alloys of Iron and Chromium, vol. II, Kinzel et al., 1940, McGraw-I-Iill Co., New York, N.Y., pp. 182, 183, 230-235, 244-249 and 258.

10 Chromium in Cast Iron, Electro Metallurgical Co., 1939,

pp. 29-37 and 42.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

